The present invention generally relates to control systems, and more particularly to a detection circuit used in a control system for detecting a state of an object that is under control.
Microcomputers are used in various control systems for detecting the state of an object that is under control and for controlling the same based upon the information about the state thus detected. In the driver circuits of actuation devices such as electric motors or magnetic relays, for example, the state of the actuation device is detected by a detection circuit, and the output of the detection circuit indicative of the state of the actuation device is sent to a microcomputer for subsequent control.
Generally, the detection circuit is driven by a supply voltage that is different from the supply voltage that powers the actuation devices. For example, actuation devices are powered by a supply voltage of 12 volts or 24 volts in the automotive applications, while the detection circuit is usually powered by a supply voltage of 5 volts. Thereby, there frequently occurs a case wherein the detection circuit has to detect a voltage that exceeds the detection circuit supply voltage.
In order to guarantee a normal operation of the detection circuit, the detection circuit usually uses a voltage divider for reducing the voltage that is to be detected. On the other hand, the use of the voltage divider raises a problem of reduced sensitivity of the detection circuit, as the variation of the voltage to be detected is demagnified by the use of the voltage divider. Thereby, the detection circuit tends to be vulnerable to noises and the risk of an erroneous operation of the detection circuit increases.
FIG. 1 shows an example of a control system that controls an actuation device by a microcomputer.
Referring to FIG. 1, the actuation device includes a solenoid L powered by a supply voltage V.sub.DD at a power supply line and a drive circuit 1.sub.A that drives the solenoid L via a MOS transistor Tr, which in turn is connected in series to the solenoid L between the power supply line and the ground. The solenoid L may be used for actuating a mechanical device such as a brake of an automobile. Thereby, the solenoid L, the MOS transistor Tr, and the drive circuit 1.sub.A form a system 1 that is subjected to control, and there is provided a microcomputer (MPV) 2 for controlling the system 1. There, the microcomputer 2 monitors the state of the system 1 such as the state of the solenoid L and controls the same in response to the monitored state of the system 1, based upon an external control signal supplied to the microcomputer 2. In order to monitor the state, a detection circuit to be described below is used.
FIG. 2 shows the construction of a most fundamental detection circuit 10 that is used in the system 1 of FIG. 1.
Referring to FIG. 2, the detection circuit 10 includes a comparator 3.sub.B having an inverting input terminal connected to a node A, where the solenoid L and the transistor Tr are connected in series, and a non-inverting input terminal connected to a reference voltage source 3.sub.A. There, the comparator 3.sub.B is activated by a supply voltage V.sub.CC and produces an output voltage S when the voltage level at the node A has decreased below the reference voltage. This output voltage S is supplied to the microprocessor 2 of FIG. 1. The microprocessor 2, in turn, supplies a control voltage to a gate of the MOS transistor Tr that forms the drive circuit 1.sub.A.
It should be noted that the detection circuit 10 of FIG. 2 operates only when the supply voltage V.sub.DD is approximately equal to or lower than the supply voltage V.sub.CC. When the voltage V.sub.DD exceeds the voltage V.sub.CC, there occurs a case wherein a voltage at the node A exceeds the supply voltage V.sub.CC. In such a case, the comparator 3.sub.B and hence the detection circuit 10 no longer operates properly.
In order to overcome this problem and to provide a detection circuit that can operate in a wide voltage range, a detection circuit shown in FIG. 3 is proposed, wherein the circuit includes a voltage divider 4 that in turn includes a first resistor R.sub.11 and a second resistor R.sub.12 that are connected in series. Thereby, the voltage at the node A is voltage-divided by the resistors R.sub.11 and R.sub.12 to a level that does not exceed the supply voltage V.sub.CC of the comparator 3.sub.B. Thereby, the circuit of FIG. 3 can operate properly even when the voltage level of the supply voltage V.sub.DD exceeds the voltage level of the supply voltage V.sub.CC.
FIG. 4 shows another conventional example of a detection circuit for use in the control system of FIG. 1, wherein there is provided another voltage divider 5 for producing a reference voltage for the comparator 3.sub.B from the supply voltage V.sub.DD.
Referring to FIG. 4, the voltage divider 5 includes first and second resistors R.sub.13 and R.sub.14 that are connected in series between the voltage supply line carrying the supply voltage V.sub.DD and the ground. Thereby, a reference voltage V.sub.REF is produced across the resistor R.sub.14 by the voltage-dividing of the supply voltage V.sub.DD, and the comparator 3.sub.B becomes immune to any voltage variation in the supply voltage V.sub.DD. In each of the circuits of FIG. 3 and FIG. 4, the operation of the comparator 3.sub.B is held normal even when the supply voltage V.sub.DD exceeds the supply voltage V.sub.CC. Thereby, the circuit is suitable for use in the car-borne or other vehicle-carried control systems.
In the detection circuit of FIG. 3 or FIG. 4, it should be noted that the variation of the voltage at the node A is demagnified as a result of the voltage-dividing. In other words, the extent of variation of the voltage at the node A is reduced when the voltage variation is detected by the comparator 3.sub.B. On the other hand, the sensitivity of the comparator 3.sub.B remains the same. Thereby, there occurs a problem in that the S/N ratio of the detection circuit is poor and the control system 1 is vulnerable to the noises that are superposed on the supply voltage V.sub.DD. It should be noted that the detection circuit should be able to detect any deviation of the voltage at the node A from a predetermined, designed level. This is particularly important when detecting the anomaly that may be caused by a leak of the current in the solenoid. When such an anomaly is overlooked, for example in the brake control system or other essential control systems of vehicles, a disastrous event may be caused.